The present invention relates generally to a technique for improving the transmission of high frequency signals within a substrate and, more particularly, to a technique for minimizing losses and skin effect and increasing propagation speed with a particular stripline and dielectric layer configuration.
The present state of the art in multi-layer substrates is to sandwich a conventional trapezoidal stripline between two substrate layers, which are generally dielectric layers.
Each substrate layer has an associated loss tangent, which indicates the amount of signal loss due to the dielectric effects of the substrate layers that are adjacent to the stripline. At high data transmission rates, the apparent loss tangent increases, thereby causing more loss. One method for reducing loss proposes reducing the surface area of contact between the substrates and the stripline. U.S. Pat. No. 4,521,755 discloses such a method for reducing the above-described losses by creating a suspended substrate stripline. The patent discloses a cylindrical air channel drilled in a solid block of conductive material in which a dielectric substrate is suspended by diametrically opposing triangular notches carved lengthwise down the channel. The block of conductive material forms the outer or return path conductor. A stripline on the top of the suspended substrate and a matching stripline on the bottom of the suspended substrate are located centrally on the suspended substrate and are connected by periodic conductive vias through the suspended substrate to form the inner conductor. The striplines may have castled edges to increase capacitance between the inner and outer conductors and hence decrease the characteristic impedance of the transmission line.
A major shortcoming of the above-identified solution is the impracticality of constructing such a structure on the small scale required on a printed circuit board (PCB). A further shortcoming is the difficulty of fabricating such a structure in an economically feasible manner. An additional difficulty is that although each stripline sees air on one broad side, each stripline still sees a substrate, which has a higher loss tangent than air, such that the transmission line has more loss than would occur if air could border the transmission line on both sides.
U.S. Pat. No. 5,712,607 discloses a transmission line with an inner conductor similar to that in the above-described patent, incorporated in a multi-layered PCB. Although this incorporation improves the manufacturing process, a notable shortcoming of this configuration is that each half of the stripline still sees a high-loss dielectric substrate on one side and hence signal integrity and signal reach suffer at higher data rates and for longer transmission paths. Furthermore, U.S. Pat. No. 5,712,607 offers no accommodation for differential pairs of striplines in which one line carries the positive polarity of the signal while the other line carries the negative polarity of the signal. These differential configurations are advantageous for noise or common mode rejection.
An additional problem inherent in the prior art results from a phenomenon known as xe2x80x9cskin effectxe2x80x9d. When current is passed through a conductor with a cross-sectional shape as shown in FIG. 13A, current tends toward the outside of the conductor thereby creating a skin as shown in FIG. 13B. The cross-sectional area of the skin is less than the cross-sectional area of the conductor, thereby creating additional losses.
The current distribution within the skin portion (Iskinxe2x80x94depth) in FIG. 13B is related to the total current I of the conductor by the equation:
Iskin depth=I/exe2x80x83xe2x80x83(1)
where e is the natural constant, which is approximately 2.718.
The skin effect worsens as the frequency of the transmitted signal increases as shown by the following equation:
xe2x80x83skin_depth=1/(fxcfx80"sgr"xcexc)xe2x80x83xe2x80x83(2)
where f is equal to signal frequency, "sgr" is equal to the conductivity of the transmission medium and xcexc is equal to the permeability of the medium. Accordingly, higher data rates lead to smaller skin depth, which in turn leads to higher losses.
Similarly, skin effect is accentuated by corners of a stripline. More current flows in the corners of the stripline than in other portions. The location of the greatest current density is where the largest losses occur, thus resulting in large losses in the corners of a conductor.
In view of the foregoing, it would be desirable to provide a technique for improving signal reach and signal integrity when using high bit rates or high signal frequencies which overcomes the above-described inadequacies and shortcomings. More particularly, it would be desirable to minimize the losses due to non-uniform current distribution and high dielectric constants. Accordingly, it would be desirable to provide a technique for manufacturing a multi-layer substrate for improving signal reach and signal integrity in an efficient and cost effective manner.
According to the present invention, a technique for improving signal reach and signal integrity when using high bit rates or high signal frequencies is provided. In one embodiment, the technique is realized by providing a multi-layer substrate comprising a conductor, a first spacer layer on a first side of the conductor, the first spacer layer having an air channel substantially coextensive with the conductor, and a second spacer layer on a second side of the conductor, the second spacer layer having an air channel substantially coextensive with the conductor.
In accordance with other aspects of the present invention, a multi-layer substrate is provided that comprises a conductor having a continuous main path and discrete spaced edges protruding substantially perpendicularly from opposing edges of the continuous main path. A first spacer layer is disposed on a first side of the conductor, the first spacer layer having an air channel substantially coextensive with the continuous main path and a solid portion overlapping with the discrete spaced edges. A second spacer layer is provided on a second side of the conductor, the second spacer layer having an air channel substantially coextensive with the continuous main path of the conductor and a solid portion overlapping with the discrete spaced edges.
In accordance with further aspects of the present invention, a conductor is provided within a multi-layer substrate. The conductor comprises a continuous main path having a first elongated edge, a second elongated edge, and a first end and a second end perpendicular to the first and second elongated edges. Additionally, the conductor may comprise a first set of discrete spaced edges protruding the first elongated edge at a first set of locations and a second set of discrete spaced edges protruding from the second elongated edge at a second set of locations. The first set of locations and the second set of locations may be selected such that a width of the conductor is constant over its length.
Alternatively, the first set of distances and the second set of distances can be identical such that the width of the conductor alternates between a minimum and a maximum width.
In accordance with still further aspects of the present invention, a method is provided for constructing a suspended stripline within a multi-layer substrate. The method comprises forming a first substrate with a conductor on one side, etching the conductor into striplines, and applying a second substrate over the etched striplines. The method further comprises forming an air channel in the first substrate and applying a third substrate to the first substrate. The method further comprises forming a channel in the second substrate and applying a fourth substrate to the second substrate.
The present invention will now be described in more detail with reference to exemplary embodiments thereof as shown in the appended drawings. While the present invention is described below with reference to preferred embodiments, it should be understood that the present invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional implementations, modifications, and embodiments, as well as other fields of use, which are within the scope of the present invention as disclosed and claimed herein, and with respect to which the present invention could be of significant utility.